Again, there are too many songs in this world with are unbelievably bad, annoying, upsetting, etc.
My spontaneous idea when I heard " least favourite" :
"Umbrella" by Rihanna
Hate the song, hate her voice which sounds more like a rusty watering can and hate her whole attitude. And what makes it the worst: the fact you hear it once and then it is stuck in your mind for the whole day...ella, ella, ella, ey, ey, ey...arrrrrgh!